In high density systems for interconnecting integrated circuits, thin (e.g. 0.2 micron to 12 micron) copper or other metallic features are sandwiched between nonmetallic layers, such as polyimide layers, which are cured in situ so as to form multilayer systems. As formed on the underlying layers, the metallic features are spaced by gaps, which may be no wider than about one micron. Since the metallic features tend to corrode excessively when exposed to precursor materials for the overlying layers, particularly at high curing temperatures, to chemical solutions, or to oxidizing atmospheres, it is necessary to provide corrosion protection of the metallic features.
Electroless nickel plating and electroless cobalt plating have been considered for providing corrosion protection on such metallic layers. See Kaja et al. U.S. Pat. No. 5,382,447. See, also, Zeller U.S. Pat. No. 4,770,899. See F. Pearlstein, "Electroless Plating," which is Chapter 31 of F.A. Lowenheim, Ed., Modern Electroplating, Third Edition, Wiley-Interscience, New York (1974) at pages 710-730, for a general exposition of electroless plating.
Usually, in electroless nickel plating as known heretofore, an aqueous plating bath containing a nickel salt, such as nickel sulfate or nickel chloride, a hypophosphite, such as sodium hypophosphite, and a salt of an organic acid, such as sodium citrate or sodium acetate, is employed. Acting as a reducing agent to convert nickel to its elemental form, the hypophosphite contributes phosphorus to the plated metal, which thus is a nickel-phosphorus alloy containing a major part of nickel and a minor part of phosphorus. The salt of an organic acid acts as a buffering agent and as a mild complexing agent for nickel. Accelerating agents, which are called exaltants, may be added to the aqueous plating bath. Stabilizers and brighteners also may be added.
Although nickel-phosphorus alloys are know to be highly corrosion resistant, electroless plating as known heretofore tends to deposit such alloys too rapidly, so as to cause extraneous plating in the gaps between such metallic features. Additional processing, such as ex situ ion beam scribing, may be then necessary to remove extraneous plating from the gaps between such metallic features, particularly if the gaps are narrower than about 500 microns, so as to avert short circuiting between the plated features.
A need has existed for an improved method for providing corrosion protection of metallic features on a nonmetallic layer, such as a polyimide, ceramic, or glass layer, or on a semiconductor layer, such as a silicon layer, without needing to employ such additional processing.